The subject matter disclosed herein relates to the field of seals used in turbomachinery. More particularly, the subject matter disclosed herein relates to a progressive clearance labyrinth seal for application at the interface of a rotating component, such as a rotor in a turbine or compressor, and a stationary component, such as a casing or stator.
Labyrinth seals used in gas turbines, steam turbines, aircraft engines, compressors, and other turbomachinery systems are susceptible to leakage because a rotor clearance may be configured to be large enough to help prevent the rotor from rubbing against the seal. If the rotor does contact the seal, which is referred to as rotor-rub, the seal may be damaged creating an even larger clearance thereafter. Specifically, rotor-rub may occur in a gas turbine during a number of rotor transients that may include rotor dynamic excitation, relative thermal distortion of the rotor and stator, or shift in the center of the rotor because of development of a hydrodynamic lubricating film in the journal bearings with increasing speed. Deflection may occur when a gas turbine passes through critical speeds, such as during start-up. Distortion may be caused by thermal discrepancies between different components within the gas turbine. A large clearance between the seal and rotor is needed because a labyrinth seal may be unable to adjust its clearance during the rotor transients as it may be rigidly coupled to the stator. The clearances between rotating and stationary components of gas turbines may affect both the efficiency and performance of the turbine. In the design of gas turbines, close tolerances between components may result in greater efficiency. Similar rotor transients occur in other turbomachinery systems such as steam turbines, aircraft engines, or compressors, and the transients may often be difficult to predict.
In addition, labyrinth seals may be configured with a Variable Clearance Positive Pressure Packing (VCPPP) ring that biases the labyrinth seal away from the rotor to a large clearance by means of a spring. This helps prevent a rotor-rub during start-up rotor transients. When the differential pressure across the seal builds up beyond a certain value, the forces on the VCPPP ring cause it to close to a small rotor clearance. In the VCPPP ring design, there exists a steam-seal joint where the VCPPP ring contacts the casing or stator. The friction at this joint may introduce a hysteresis in the opening and closing of the VCPPP ring. If there are rotor transients after the VCPPP ring has closed, there will be rotor-rubs and damage to labyrinth teeth.